The present invention relates to electrophotographic printing. It finds particular application in conjunction with a method and system for controlling a printing device""s tone reproduction curve (TRC). The invention helps minimize contouring and maximize a number of shades or colors available for an output image. The invention will be described in reference to a xerographic print engine. However, the invention is also. amenable to other electrophotographic processes, such as for example, ionographic print engines and like applications.
Electrophotographic copiers, printers and digital imaging systems typically record an electrostatic latent image on an imaging member. The latent image corresponds to the informational areas contained within a document being reproduced. In xerographic systems, a uniform charge is placed on a photoconductive member and portions of the photoconductive member are discharged by a scanning laser or other light source to create the latent image. In ionographic print engines the latent image is written to an insulating member by a beam of charge carriers, such as, for example, electrons. However it is created, the latent image is then developed by bringing a developer, including colorants, such as, for example, toner particles into contact with the latent image. The toner particles carry a charge and are attracted away from a toner supply and toward the latent image by an electrostatic field related to the latent image, thereby forming a toner image on the imaging member. The toner image is subsequently transferred to a physical media, such as a copy sheet. The copy sheet, having the toner image thereon, is then advanced to a fusing station for permanently affixing the toner image to the copy sheet.
The approach utilized for multi-color electrophotographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color complimentary thereto and the process is repeated for differently colored images with the respective toner of complimentary color. Thereafter, each color toner image can be transferred to the copy sheet in superimposed registration with the other toner images, creating, for example, a multi-layered toner image on the copy sheet. This multi-layer toner image is permanently affixed to the copy sheet in substantially conventional manner to form a finished copy.
An image to be rendered (an input image) is received in the form of, or is transformed into the form of, a set of contone values. For example, each contone can have a value ranging from 0 to 255 (in eight bit systems) or from 0 to 4095 (in higher resolution twelve bit systems). The contone values are indicative of how much colorant should be applied to the output medium in order to render a small portion of the image. For example, zero may indicate that no colorant should be applied to a small portion of the medium and a contone value of 255 may indicate that the entire area associated with a halftone cell should be covered with toner. Often, an ideal relationship between contone values and the amount of colorant applied to the medium is a linear one. That is, typically an ideal or target tone reproduction curve (TRC), which relates input contone values to, for example, colorant density applied to the print medium, relates each possible contone value to a unique and incrementally proportional amount of colorant.
Some electrophotographic systems include a hierarchical control scheme in an attempt to provide an actual tone reproduction curve (TRC) that is as close as possible to the ideal or target tone reproduction curve (TRC). For example, some electrophotographic systems include what are referred to as level 1 control loops for maintaining electrophotographic actuators at associated set points, level 2 control loops for selecting set points for the level 1 control loops, and level 3 controls for compensating for residual differences or errors between the actual TRC and the target TRC in spite of the efforts of the level 2 control loops.
Xerographic actuators include, for example, cleaning field strength or voltage, development field strength or voltage, imager or laser power, and AC wire voltage associated with some developers. For example, in some xerographic environments level 1 control loops include electrostatic voltmeters (ESV) for measuring charge voltage generated by charge applied to a photoconductive member. For instance, the ESV measure the charge applied in the area of test patches in inter document or inter page zones (IPZ) of the photoconductor. If measured voltages, such as, for example, a discharged area voltage, or a cleaning voltage of an area surrounding a discharged area deviate from set point values, level 1 control loops adjust xerographic actuators to return the measured voltages to set point potentials. For example, the level 1 control loops vary a charge or bias voltage applied to elements of a developer to adjust a resulting development field and/or cleaning filed. Additionally, the level 1 control loops may adjust a laser power to return a related discharge field back toward a discharge field set point.
Level 2 control loops include, for example, infrared densitometers (IRD). In xerographic environments, and perhaps in other electrophotographic environments, infrared densitometers are also known as Enhanced Toner Area Coverage Sensors (ETACS). The infrared densitometers or ETACS are used to measure, for example, the density of toner or colorant applied to or developed on the photoconductive member. For instance, a set of test patches is written in an interdocument or interpage zone on the photoconductor. The test patches are developed and the amount or density of colorant or toner present in the test patches is measured. If the amount of colorant or toner in a test patch is incorrect or varies from a target test patch density, the level 2 control loops generate or select one or more new set points for the xerographic actuators of the level 1 control loops.
For instance, if a high-density test patch, such as a test patch corresponding to a target density of 100 percent (e.g., contone value 255), includes too little colorant or toner (is less dense than the target density), then the level 2 control loop may increase a set point related to the generation of a development field.
If the measured or actual density of a low-density test patch, or a test patch associated with a low-target density, such as, for example, 10 percent (e.g., a contone value of 25 or 26), includes more colorant or toner than is indicated by the associated target density, the level 2 controls may select or determine a new set point for a level 1 control loop associated with controlling a cleaning field voltage. For instance, increasing the cleaning field may reduce a toner density measured in a next low-density test patch.
If an infrared densitometer measures a deviation from a midrange target density in an associated test patch, the level 2 controls may select or determine a new set point for a level 1 controller responsible for regulating laser power.
The level 2 control loops strive to maintain the actual densities of test patches at desired or target levels. The assumption is that by adjusting the level 1 actuator set points to maintain the densities of a few test patches at target levels, an entire actual TRC will be maintained at or near an ideal or target TRC.
However, due to environmental and system changes, such as, for example, temperature, humidity, system age, wear, thermal expansion and contraction, toner quality and toner sources, the actual TRC of a system can become nonlinear. Therefore, anchoring an actual TRC to an ideal or target TRC at a few points, such as the high, low and midrange target densities described above, does not always maintain the entire actual TRC at ideal or target levels.
For example, referring to FIG. 1, even though the level 2 controls adjust level 1 set points in order to anchor an actual TRC 114 to a target TRC 118 at a high 122, low 126 and midrange 130 points on the target 118 TRC, the actual TRC 114 can meander away from the target TRC 118 in a first 134, second 138 and third 142 regions between the high 122 and midrange 130, midrange 130 and low 126, and low 126 and origin 146 points of the target 118 TRC, respectively.
Errors or deviations from the target TRC 118 of the actual reproduction curve 114 lead to errors in gray scale or color of images in output documents. For example, a portion of an input image is described in terms of the target or ideal TRC 118. The image portion is described as being associated with an input 150 contone value of 210. If the actual TRC 114 coincided perfectly with the target reproduction curve 118, then the input 150 contone value 210 would correspond to an output 154 contone value (and related toner density) of 210. However, due to temperature, humidity, system wear, variations in toner specifications and the like, the exemplary actual TRC 114 does not exactly coincide with the target TRC 118 and a residual error 158 exists between the actual TRC 114 and the target TRC 118. The error 158 in the actual TRC 114 causes a related gray scale or color error 162, and the input 150 contone value 210 results in a printed color or gray scale (toner density) 166 related to an ideal contone value of about 225.
Color errors caused by residual differences in actual TRCs and target TRCs, such as the one described above, are undesirable and can be unacceptable in some reprographic applications. Therefore, as mentioned above, some electrophotographic systems include a third level of control. Level 3 control loops may share the infrared densitometers of the level 2 control loops. Alternatively, level 3 control loops can include other sensors.
To implement level 3 control, a plurality of additional test patches are developed in inter page zones of an imaging member. The plurality of level 3 test patches is associated with a plurality of target level 3 test patch densities. The plurality of target level 3 test patch densities may or may not include the high 122, low 126 and midrange 130 target test patch densities described above. For instance, the plurality of target test patch densities includes test patch densities within the first 134, second 138 and third 142 regions of the actual and target TRCs 114, 118. Sensing the actual densities of the level 3 test patches associated with the plurality of target level 3 test patch densities provides the level 3 controllers with information about the actual TRC and, therefore, about the residual error within the regions 134, 138, 142 between the level 2 control points 122, 130, 126 and origin point 146. The level 3 controls use this information to build color correction lookup tables to be used in an image path of the system.
While color correcting lookup tables improve color accuracy, that improvement comes at the expense of a smoothness in color transition and a reduction in a number of available colors or shades. This loss can result in noticeable borders between regions in an image having slightly different color instead of smooth, blended transitions therebetween. The side effects of color correction lookup tables are referred to as banding and contouring.
Clearly, banding and contouring are undesirable. Therefore, there is a desire for a method that reduces the need for and/or the side effects of color correction lookup tables.
The present invention contemplates a new and improved system and method which overcomes the above-referenced problems and others. One aspect of the invention is a method operative to control an electrophotographic system. The method comprising measuring an actual tone reproduction curve; comparing the actual tone reproduction curve to a target tone reproduction curve, and changing at least one point at which the actual tone reproduction curve is controlled to minimize error in the actual tone reproduction curve.
For example, changing at least one point at which the actual tone reproduction curve is controlled can include changing a target density of a control patch.
Changing a target density of a control patch can include changing a point along the actual tone reproduction curve at which the actual tone reproduction curve is anchored to the target tone reproduction curve by a control effort of a developability control loop.
One embodiment of the invention includes a method operative to minimize contouring in an electrophotographic system. The method can include selecting a target tone reproduction curve, selecting target level 2 default test patch densities, selecting target level 3 test patch densities for measuring an actual tone reproduction curve, developing level 2 default test patches based on the selected target level 2 default test patch densities, measuring actual level 2 test patch densities, determining a level 2 error between the actual level 2 test patch densities and the target level 2 default test patch densities, adjusting at least one level 2 actuator based on the determined level 2 error, developing level 3 default test patches based on the selected target level 3 default test patch densities, measuring actual level 3 test patch densities, determining a level 3 error between the actual level 3 test patch densities and the target level 3 test patch densities, determining a new target level 2 test patch density based on the determined level 3 error, developing the new level 2 test patch based on the new level 2 target test patch density, measuring a new actual level 2 test patch density, determining a new level 2 error between the new actual level 2 test patch density and the new target level 2 default test patch density, and adjusting the at least one level 2 actuator based on the determined new level 2 error.
Determining a level 3 error can include fitting a curve to the measured actual level 3 test patch densities and comparing the fitted curve to the target tone reproduction curve.
Determining a new level 2 target test patch density can include selecting a new level 2 target test patch density so as to reduce an area between a measured tone reproduction curve and an actual tone reproduction curve. Selecting a new level 2 target test patch can include attempting to make a first average area, between a first region of the actual tone reproduction curve and the target tone reproduction curve, equal to a second average area, between a second region of the actual tone reproduction curve and target tone reproduction curve. Additionally or alternatively, selecting a new level 2 target test patch can include attempting to make a third average area, between a third region of the actual tone reproduction curve and the target tone reproduction curve, equal to the second average area, between the second region of the actual tone reproduction curve and the target tone reproduction curve.
An embodiment of an electrophotographic operative to carry out the methods of the invention can include a level 1 control loop for maintaining a developability actuator at a developability actuator set point, a level 2 control loop for assigning a value to the developability set point based on a first set of system performance measurements, thereby providing a first order correction, and means for optimizing an aspect of the first set of system performance measurements, thereby minimizing an aspect of error in an actual tone reproduction curve.
Some embodiments of the electrophotographic system include a level 3 control loop for providing image path corrections based on a second set of system performance measurements, thereby providing a second order correction, the actions of the means for optimizing an aspect of the first set of system performance measurements being operative to minimizing the corrections provided by the level 3 control loop and/or side effects thereof.
For example, the level 1 control loop can include a development voltage control loop for maintaining a development voltage at a development voltage set point, a voltage control loop for maintaining a cleaning voltage at a cleaning voltage set point and/or a laser power control loop for maintaining a laser power at a laser power set point.
The level 2 control loop can include a colorant density control loop operative to adjust at least one of a bias voltage, a cleaning voltage, a laser power and a wire AC voltage, in order to maintain an actual colorant density at a target colorant density.
The level 3 control loop can include a colorant density control loop operative to adjust image contone values in order to maintain an actual colorant density at a target colorant density.
The means for optimizing an aspect of the first set of system performance measurements can include a means for selecting the target colorant density so as to reduce a control effort needed from the level 3 control loop. Alternatively or additionally, the means for optimizing an aspect of the first set of system performance measurements include a means for selecting the target colorant density so as to reduce a contouring effect resulting from control efforts of the level 3 control loop.
The first set of performance measurements can include a first set of measured control patch densities, the measured control patch densities being related to target control patch densities. The level 2 control loop can be directed toward aligning an actual tone reproduction curve with a target tone reproduction curve at at least points on the respective curves related to the target control patch densities. The means for optimizing an aspect of the first set of system performance measurements can include a processor and a procedure performed by the processor. The procedure can include comparing the target tone reproduction curve to a measurement of the actual tone reproduction curve, determining a sign of a desirable change in at least one target control patch density based on the comparison of the target tone reproduction curve and the actual tone reproduction curve, determining a magnitude of the desirable change in the at least one target control patch density based on the comparison of the target tone reproduction curve and the actual tone reproduction curve, and changing the at least one target control patch density based on the determined sign and the determined magnitude.
In some embodiments the level 1 controller is operative to maintain a xerographic actuator at a developability actuator set point.